Process for bending glass sheets using fluid barrier

ABSTRACT

A process for bending a glass sheet according to which the glass sheet is heated in horizontal position until its bending temperature is reached and is brought into contact with an upper bending form by a force of a pneumatic nature. A fluid barrier is generated at the periphery of the glass sheet, the fluid barrier limiting the entry and exit of air near the upper bending form.

This is a continuation of application Ser. No. 07/709,207, filed on Jun.3, 1991, now U.S. Pat. No. 5,135,558.

FIELD OF THE INVENTION

The invention relates to techniques for production of bent, andoptionally tempered, glazings generally intended for equipping motorvehicles. More precisely, the invention relates to such techniques inwhich each glass sheet is heated in a horizontal position until itsbending temperature is reached, is brought into contact with an upperbending form by a force of pneumatic nature, for example by suction, isshaped by application against this upper bending form and/or by a lowerbending form and/or other bending tools, and finally undergoes acontrolled cooling, for example of the heat tempering type.

DESCRIPTION OF THE RELATED ART

According to these techniques, the glass sheet goes through a heatingfurnace on a bed of drive rollers which brings it into a bendingstation. In the latter, the glass sheet is lifted above the roller bedby a pneumatic force. The rise of the glass sheet is completed oncontact with an upper bending form whose curvature it assumes, theshaping optionally being supplemented, for example, by a shaping byinertia and gravity due to dropping on a lower bending form and/or apressing.

The pneumatic force acting on the glass sheet, in a first variant ofthese techniques, is generated by a suction through the upper formprovided for this purpose with suction orifices (EP-B-3 391). But if thepartial vacuum exerted is too strong, these orifices cause a marking ofthe glass sheet. When the initial distance between the orifices and theglass sheet is large, it is necessary to increase the level of thepartial vacuum to succeed in causing the lift-off of the glass. Inpractice, this technique therefore is suitable only if the entire face(turned toward the glass) of the upper form can be lowered very close tothe surface of the glass sheet, which initially assumes a plane orbarely curved form.

In a second variant, known particularly from patents or patentapplications FR-B-2 085 464, EP-A-240 418, EP-A-241 355 and EP-A-255432, the pneumatic force acting on the glass sheet is obtained by aperipheral suction. This technique of taking up the glass sheet isadvantageously polyfunctional, i.e., it can be used with a great numberof bending processes such as, for example, processes comprising droppinga glass sheet on a frame open in its center, or deformation against anupper bending form and/or by pressing. Further, the optical qualityobtained is particularly great because the contacts between the glasssheet and the bending tools are relatively limited, the glass sheet notbeing either lifted mechanically (because of the peripheral suction) orapplied at its central part against the upper bending form, which avoidsformation of optical defects in this zone. To this high optical qualityare added simple and convenient replacement of the covering that coversthe upper bending form (which does not have to be pierced) and thepossibility of working with an upper bending form whose dimensions aresmaller than those of the glass sheet, so that possible enameled partsof the glazing are not in contact with the upper bending form.

To create a sufficient pressure difference between the two faces of theglass sheet, there is constituted a chamber closed on top by the upperbending form, below by the glass sheet and on the sides by the lateralwalls or wall portions of a bottomless box also called a skirt, thischamber being depressurized, for example, by an air jet pump. To avoidmarking the glass, this skirt should be slightly separated from theglass sheet and a leak necessarily results, which cannot be reduced evenif the skirt is shaped. But this leak by definition generates a loss ofefficiency of the suction and constitutes the main limitation to theintensity of the force acting on the glass sheet, particularly at themost critical moment of the process, i.e., during lift-off of the glasssheet.

It has been proposed in a first step, in FR-B-2 085 464, to use shapedskirts to effectively channel the lateral leaks and to bring the skirtas low as possible during taking up of the glass sheet. However, thistechnique is not without a drawback: first of all, it requires a veryfine adjustment of the height position of the skirt, an adjustment madedifficult by the phenomena of expansion of the tools due to the hightemperature prevailing in the bending station and yet essential to keepthe skirt and/or the bending form from striking and marking the glasssheet. Further, the skirt is in the way during replacement of a bendingform or replacement of the paper or fabric insert used to cover thesurface of contact of the upper form with the glass.

On the other hand, practice has shown that beyond a certain complexityof glazing forms, it is not possible to obtain a perfect application ofthe glass sheet on the upper form simply by suction. One of thesolutions already proposed for this problem is to supplement thepneumatic application by a mechanical pressing by a pressing frame, butthen the presence of the bottom of the skirt hinders introducing thispressing frame under the glass sheet. The skirt also creates a totallack of visibility which is very harmful during adjustments of the formand/or of the pressing time. Further, the pressing frame and equipmentthat is joined with it should be small enough to be housed under theskirt which prevents a pressing by a tempering frame. Finally, in thecase of dropping the glass sheet on a bending and tempering frame, openin its center, the skirt imposes a relatively large drop height: theinertia effect of the drop is therefore greater, hence a danger ofdouble bending, i.e., of obtaining undesired curves. Moreover, thegreater the drop height, the more problems of drop delay (dueparticularly to gluing effects of the enamels) generating difficulties,for example, linked to the poor positioning of the glass sheet on theframe and especially curve defects when the glazing does not fall flatbut more or less crosswise.

In patent applications EP-A-241 355 and EP-A-240 418, it was proposed tooperate with a short skirt, leaving a part of the bending form visible.This solution makes it possible to eliminate most of the above-mentioneddrawbacks, but it is essentially limited to the use of upper bendingforms exhibiting a less pronounced curve, without which the initialsuction cannot be sufficient to make the glass sheet separate from theroller bed. To remedy this, patent application EP-A-241 355 discloses adouble-wall skirt, the outside walls being retractable to allow theintroduction of a pressing counterform under the glass sheet, but thislatter solution does not make it possible to work with all types ofpressing frames because of the smallness of the space under the skirt,smallness which excludes the use of a process involving dropping of theglass sheet (because such a process requires a relatively large frame)and, moreover, the absence of visibility is always total.

Finally, in a third variant of these forming techniques, the glass sheetis lifted and applied against the upper form by a rising hot gascurrent. But there again it is relatively difficult to exert asufficient force to apply the sheet correctly on the curved surface, thejets directed toward the central part of the sheet having the tendency,by draining off on the sides, to deflect toward these sides the lateraljets directed toward the marginal areas of the glazing so that theselateral jets have a reduced impact force, which force is already reducedby the fact that the jets do not act essentially perpendicularly to theupper form but with a certain angle relative to the perpendicular due tothe curve of the upper form.

All these known approaches of the art are in practice largelysatisfactory with most of the usual sheets. But for those that exhibitthe most complicated shapes, a good quality is often obtained only atthe price of long adjustments or technical compromises which do notallow the operator as great a freedom of action as he could desire.

SUMMARY OF THE INVENTION

The invention has as an object a bending process and device includingthe taking up the glass sheets, making possible the simultaneousexertion of a great force at the moment of lift-off, a correctapplication against a possibly very greatly curved upper bending form,the introduction under the upper bending form of tools for bendingand/or recovery of glass sheets and finally a good visibility of theglass sheet during its treatment.

This object is achieved according to the invention by a bending processaccording to which the glass sheet is heated in horizontal positionuntil its bending temperature is reached and is brought into contactwith an upper bending form by a force of pneumatic nature, a fluidbarrier being generated on the periphery of the glass sheet, said fluidbarrier channeling the gases exerting said pneumatic force and limitingthe entry of outside air near the upper bending form.

The fluid barrier generated just at the periphery of the glasssheet--but not in contact with it--constitutes a sort of skirt whichhelps in taking up the sheet, which is achieved either by suctionthrough or at the periphery of the upper form or by a rising flow of hotair. It thus totally or partly replaces the mechanical skirt known inthe art, all of whose functions it advantageously provides, with thefurther advantage of perfect transparency. Depending on the complexityof the desired glazing shape, the height of the vertical transfer of theglass sheet and the presence of other means acting on the glass sheet(the three "types" of pneumatic force listed above can actually bepartially combined), the fluid barrier can cover all or part of theperimeter of the glass sheet. The fluid barrier can also be associatedwith a very short skirt which provides a slight leak at the end of theshaping operation, the fluid barrier then essentially serving during theseparation phase.

Advantageously this fluid barrier is generated by jets of compressed airpreferably coming from a ramp mounted integral with the upper bendingform, the divergence of the jets on leaving the ramp providing thecontinuity of the barrier and thus the fluid tightness essential forobtaining a "skirt" effect. Of course, this barrier should not opposethe rise of the glass sheet. Since when the chamber is being put underpartial vacuum, the air jets are drawn toward the center, it ispreferable to operate with oblique jets deviating from the glass sheetand which are then brought together by the effect of the vacuum.

According to another embodiment of the invention, the compressed airramp is mounted stationary, set in the roller conveyor conducting theglass sheets from the furnace to the bending station.

It is also possible to use slotted nozzles directly forming a continuousair layer or a lip mounted directly on a tube.

The process according to the invention is remarkable in that it issuitable for a very great number of bending processes. Thus, asindicated above, the pneumatic force acting on the glass sheet can bedue to peripheral suction of the glass sheet, to a rising hot current ora suction through the bending form for the continuation of the operationor also by a combination of these means. Moreover, it is possible tooperate with a plane upper bending form--so that the bending isperformed by dropping on a lower bending form generally performing thefunction of a tempering frame--or with a curved upper bending form--sothat all or part of the bending of the glass is performed during theapplication against the upper bending form. In any case, the fluidbarrier according to the invention simplifies the introduction ofequipment of the tempering frame and/or pressing frame type under theupper bending form. Further, the much better visibility which resultsfrom the use of a fluid barrier helps in the interpretation of phenomenaactually occurring on contact with the upper bending form which, ifnecessary, allows a suitable modification of the adjustments of theequipment.

The "skirt" effect obtained with the fluid barrier, of course, functionsslightly differently according to the nature of the pneumatic forceacting on the glass sheet. With a suction, the fluid barrier makes itpossible to define a sort of chamber in which the vacuum is created.With a blowing, the fluid barrier will partly act as a deflector bysending the deviated blown gases toward the glass sheet. But in anycase--whether it be a very slight leak around the glass sheet or thedeflection of rising currents--the fluid barrier according to theinvention makes possible a greater "effectiveness" of the pneumaticforce which, if necessary, makes it possible to reduce its intensity. Arelatively weak suction force suffices to create a pressure differencebetween the two faces of the glazing sufficient to make the glass sheetrise. Movement of the glass sheet against the upper form can thus beperformed under particularly gentle conditions, without fear of markingthe glass. Whereas with the known devices of the art, the initialsuction must be very strong and is then reduced in the phase of shapingthe glass, with a fluid barrier according to the invention, thenecessary vacuum level at the beginning of the process is so low that,on the contrary, it is advantageous to increase the partial vacuum atthe end of the application, which allows the glass sheet to better fitthe contours of the form. More particularly, the lift-off phase can beconsidered as not absolutely critical and then the partial vacuum levelcan be selected exclusively in consideration of the shaping on contactwith the upper form.

A very advantageous example of application of the invention is that ofthe production of moderately bent glazings which will take advantage ofthis increased efficiency of the pneumatic force. Thus in numerous casesthe limit beyond which a mechanical pressing proves necessary will beslightly extended, the sheet being bent simply by the pneumatic forceapplying it against the curved upper bending form.

Even if the shape is not entirely achieved by this pneumaticapplication, the bending difference to be made up by mechanical pressingis still reduced so that the pressing can be performed, leading to aminimum of sliding. Under these conditions, it can then be considered toproceed to the slight pressing by the tempering frame which obviously isadvantageous from the viewpoint of the rate of production and equipmentcost. In this case, it is important to cover the tempering frame with afabric not promoting marking too much, such as taught by European patentapplication No. 312 439.

It has further been noted that this air current at high speed creates a"venturi" effect. The partial vacuum thus generated is able to be on theorder of 10 mm of a water column, a figure approaching the value on theorder of 15 mm of a water column which is necessary to lift the glasssheet. This partial vacuum thus contributes to the lift-off andlightening of the sheet during the centering phase which generallyprecedes the suction. To benefit from this lightening during centering,the fluid barrier is generated before the intervention of the pneumaticmeans of suction and blowing starts, i.e., as soon as the glass sheetpenetrates the space located below the form.

BRIEF DESCRIPTION OF THE DRAWINGS

Other details and advantageous characteristics of the invention willcome out from the following description with reference to theaccompanying drawings which represent:

FIG. 1 is a schematic side view of a bending-tempering installation withan upper bending form surrounded by a fluid barrier, and indicating indash lines a conventional construction; and

FIG. 2 is a schematic view of a sucking bending form with suctionboosted by a fluid barrier according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the characteristic elements of the bending cell of abending-tempering installation operating essentially according to theprinciple of patent FR-B-2 085 464, except as noted below. In a waywhich is well known and therefore not shown here, the glass sheets arecharged at the input of an electric furnace on a roller conveyor whichextends to the output of the furnace. The travel of the glass sheets isstopped under an upper bending form 1, this stopping generally beingperformed in cooperation with centering means. Upper bending form 1 inthe example selected here consists of a greatly curved piece of sheetmetal corresponding to the shape it is desired to impart to glass sheet2 delivered by rollers 3. Upper bending form 1 is placed in a bottomlesssuction box 4 whose side walls form a skirt which surrounds upper form 1to define around the latter a leak space when box 4 is put in a partialvacuum by pump 5 to generate a suction force.

To make the glass sheet rise and to apply it against the upper bendingform, it is necessary that the suction force be maximum at the moment oflift-off and for the final application of the most curved parts.

According to patent FR-B-2 085 464, this is achieved by the use of a"long" shaped skirt (represented in FIG. 1 by dash lines 6). Asexplained above, this long skirt constitutes an obstacle which is hardto get around and which is in the way merely by its physical bulk.Moreover, all types of pressing frames cannot be housed inside such askirt. The solution proposed in the embodiment of the invention shown inFIG. 1 consists of a very short skirt, whose dimensions are ideal at theend of application against the upper bending form, supplemented by afluid barrier.

The fluid barrier here is generated by a ramp (e.g., conduit) 7 fed withcompressed air and provided with a series of orifices. A small airvolume, high air velocity system is advantageously used. On the otherhand, to take into account the effect of the attraction of the vacuum,the fluid barrier is initially dimensioned slightly larger than theglass sheet and angled obliquely to the vertical, aperture angle α, forexample, being between 15° and 25° or even more.

The position of the orifices of the ramp and the air pressure areoptimized to replace the conventional skirt with a fluid barrier. By wayof example, a skirt 100 mm in height may be replaced by a fluid barrierobtained from a ramp fed at a pressure of 5 bars (5·10⁵ PA), drilledwith 200 holes 1 mm in diameter and placed at a constant pitch of 5 mm.If it is desired to replace a skirt of 40 mm, it will be possible towork with a lower feed pressure, or to space the holes a little fartherwhich, for example, can be between 7 and 10 mm. The latter value shouldnot be exceeded by too much because then the spaces between two adjacentjets may be wide enough to let induced air currents pass. Compressed airfeed pressures greater than 5 bars can be used and optionally allowholes that are more widely spaced. For these purposes, it isadvantageous to have means to make the pressure of the blown compressedair vary.

It should be noted that the ramp can cover only a part of the perimeterof the glass sheet and be placed, for example, only opposite the areaswhere the distance to be traveled by the glass sheet is the greatest.

Ramp 7 is preferably mounted directly on suction box 4 and is integralwith the up and down movements of the box. For this purpose, theconnection with the central compressed air circuit is performed, forexample, by a hose. It should be noted that, given the small projectedcompressed air delivery volume and the fact that these jets are neverdirected toward the glass sheet, it is not necessary that the air bepreheated to the bending temperature. However, in order not to disturbthe thermal equilibrium of the bending station and to avoid cooling theclosest part of the glass sheet, it is preferable to provide at leastone preheating of the jets, for example, by making the feed ducts gothrough the furnace for heating the glass sheets. On the other hand, theramp is preferably placed so that the fluid barrier constitutes a realcontinuation of the bottomless suction box, preventing the penetrationof induced air between the ramp and box.

In a variant embodiment of the invention, the compressed air ramp is ina stationary position relative to the conveyor. However, thisarrangement is not preferred, since it is then more difficult to inducethe fluid barrier to extend the bottomless box in a relatively airtightmanner.

In the embodiment of FIG. 1, the pneumatic force which acts on the glasssheet is exclusively due to the peripheral suction. However, theinvention is not limited to this first type of shaping process and thepneumatic force can be generated in whole or in part by a suctionthrough the upper form itself, pierced with suction holes 8, as can beseen in FIG. 2. In this case, the fluid barrier according to theinvention contributes to weakening the value of pressure P₁, under upperbending form relative to outside pressure P₂, this being opposed to thepenetration of induced air currents. For this reason, the lift-off ofthe glass sheet is performed more easily.

As is clear from FIGS. 1 and 2, the glass sheet remains perfectlyvisible during the entire period of its application against the upperbending form. The space located under and on the side of the glass sheetis also perfectly visible, which allows the introduction of any type ofequipment and particularly of pressing or tempering frames.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. Process for bending a glass sheet comprisingthe steps of:heating the glass sheet in a horizontal position to abending temperature thereof; bending the glass sheet by bringing theheated glass sheet into contact with an upper bending form in an airatmosphere by use of a pneumatic force; and generating a fluid barrieradjacent a periphery of the glass sheet during said bending step, saidfluid barrier comprising the high pressure gas jets and limiting entryof air from an exterior of the fluid barrier at the immediate vicinityof the upper bending form,wherein the fluid barrier extends aboutsubstantially an entirety of the perimeter of the glass sheet to bebent.
 2. Process according to claim 1, wherein the pneumatic forcecomprises suction applied through the upper form.
 3. Process accordingto claim 1, wherein the pneumatic force comprises suction applied at theperiphery of the glass sheet.
 4. Process according to claim 1 whereinthe pneumatic force comprises applying a rising flow of hot air. 5.Process according to claim 1, including the step of increasing theintensity of the pneumatic force when the glass comes in contact withthe upper form.
 6. Process according to claim 1, including the step ofcentering the glass sheet under the upper bending form, wherein thefluid barrier is also generated during the centering step.